Generally, in scanning probe microscopy, a probe with a nanometer-sized tip is scanned relative to a substrate surface that is to be investigated. In one mode of operation, the so-called contact mode of operation, the tip of the probe is scanned in mechanical contact with the substrate surface. The tip performs a sensing function of the interaction forces between itself and the substrate surface at each scan point. This information is then used for mapping specific characteristics of the substrate surface by yielding an image thereof. The magnitudes of the interaction forces that are sensed by the tip are relatively small, typically on the order of 1 nN to 1 μN. In order to detect them, transduction of the interaction forces is achieved by the choice of an appropriate force sensor, for example, in local probe microscopy; the force sensor is a cantilever spring on which the probe is mounted.
The interaction forces between the tip and the substrate surface depend on their respective physical and/or chemical characteristics. For example, if the tip comprises a magnetic material, it can be applied for sensing magnetic interactions with magnetic particles present in the substrate. Alternatively, if the tip comprises a conducting material, it can be applied for sensing electrostatic interactions with the substrate surface by applying a voltage between the tip and the substrate. For the investigation of the chemical properties of the substrate surface, the tip can be functionalized by adsorbing chemically active species thereon in order to sense specific chemical interactions with the substrate surface. Alternatively, it can be made of an inert insulating material thereby facilitating the topography of the substrate surface to be mapped. This can be done by exploiting hard-core repulsion forces between the tip and the surface. The tip may also be used to perform a combination of the aforementioned sensing principles.
As mentioned earlier, the magnitude of the interaction forces between the tip and the substrate surface are small and typically on the order of 1 nN to 1 μN. However, taking into account the nanometer scale dimensions of the tip and assuming that the area of contact between the tip and the substrate surface is on the order of 1 nm2 to 1000 nm2, the magnitude of the stress that is locally exerted on both the substrate surface and the tip can reach relatively high values and may be on the order of MPa to GPa. If the tip is displaced laterally over the substrate surface, shear forces of a similar order of magnitude are generated. Shear stress is considered to be one of the main causes of irreversible and undesirable alterations of the tip and/or the substrate surface, such alterations are typically referred to as wear. Wear can be manifested in a variety of ways. The following examples include tip-blunting; modification of the substrate surface topography which may, in some cases, be the irreversible damage of the substrate surface; loss of chemical and/or physical functionality of the tip, etc. Generally, wear is associated with the unintended displacement of atoms or molecules at the tip or the substrate surface.
It is known that wear may be reduced by using an alternative mode of operation in which the tip and the substrate are not maintained in contact during scanning but instead intermittently brought into contact with each other at the fundamental resonance frequency of the support structure onto which the probe is mounted. In this so-called tapping mode of operation, the reduction in wear can be attributed to the dynamics of the interdigitated network that forms between the tip and the substrate surface on an atomic/molecular scale when they are in contact. By periodically removing the tip from the substrate surface, the interdigitated network is allowed to disentangle, thereby relaxing the shear stress that accumulates between them when they are scanned in contact with each other.
Some problems associated with the tapping mode include: the sensing of bandwidth when bandwidth is dependent on and, therefore, limited by, the resonance frequency of the support structure onto which the probe is mounted. In order to cope with the vibration amplitude of the probe, the dynamic range of the sensing channel is chosen to be broad and control of the vibration amplitude is done by specialized and complicated control-loop schemes.
Accordingly, it is a challenge to increase the lifetime and performance of probes in applications where they are operated in contact with respect to a substrate surface.